Antibiotics, Bacteria and Natural Selection
Physicians frequently prescribe antibiotics for people with bacterial infections. Many prescriptions require several doses of antibiotics
over 10 days. Patients usually begin to feel better after 3 days, and are frequently asymptomatic after 6 days. It is considered a health
risk and irresponsible use of antibiotics to stop the prescription early, before all the medication has been taken.
Predict: What will happen to the population of bacteria that are not killed by an antibiotic, and how does this relate to
evolution?
Background Vocabulary Review
A population is a group of the same kinds of organisms living in the same area. In any population, there is variation in traits among the
organisms due to mutation or the recombining of genes from egg and sperm cells. Even bacteria, which reproduce by asexual
reproduction (splitting in half), can have offspring that are not exactly the same due to mutations of the DNA or through exchanging
DNA with another bacterium. A mutation is a permanent change in the DNA sequence of a gene.
Some traits help an organism survive and some do not help survival. Some traits, like being left- or right-handed, have no effect on
survival. The environment determines which traits are favorable. Selective pressure is something that changes the ability of an
organism to survive in a particular environment. A selective pressure can be something that is living or something that is not living.
Organisms with favorable traits survive. We call the favorable trait an adaptive advantage because it helps the organism survive.
Organisms with an adaptive advantage (favorable traits) pass the genes for those traits on to their offspring. By this mechanism, over
time, more and more organisms in a population have the favorable traits. We say the population has evolved and that the organisms
have adapted to the environment.
Data collection
The effect of antibiotics on the number of normal and mutated bacteria
Time
Beginning (step 0)
Number of normal
(packing peanuts)
bacteria
8
Number of mutated
(Skittle) bacteria
2
After step 4
(first dose of antibiotic, D1)
Remaining bacteria reproduce
(first reproduction, R1)
After step 8
(second dose of antibiotic, D2)
Remaining bacteria reproduce
(R2)
After step 10
(D3)
Remaining bacteria reproduce
(R3)
Graph of your data showing the change in the number of each kind of bacteria over time.
The effect of antibiotics on the number of normal and mutated bacteria over time
Number of
bacteria
1. When you used a toothpick to pick up and remove bacteria, that step represented the antibiotics killing the bacteria.
Use numbers from your data to describe which kind of bacteria, normal or mutated, were killed most easily with every
dose of antibiotics.
2. Describe the selective pressure that determined which bacterial trait was favorable.
3. In the table below, list each of the traits in the category in which it belongs.
Normal bacteria traits:
White cylindrical soft
bodied
Mutated bacteria traits:
colorful, disc shaped,
hard bodied
Adaptive advantage
(helped survival)
Unfavorable trait
(hurt survival)
Unaffected traits
(neither helped nor hurt)
4. Choose one trait from above that was an adaptive advantage. Explain how the trait was an adaptive advantage.
5. Choose one unfavorable trait from above. Explain how the trait was unfavorable.
6. Revisit your prediction from part A. After completing this simulation, describe what happens to the bacteria that are not
killed by antibiotics and explain how this relates to evolution.
7. When a doctor prescribes an antibiotic, the doctor instructs you to take all of the medication until it is gone. This advice
should be taken very seriously. If you stop taking the antibiotics too early, what happens to the bacteria that, by chance,
have a trait that prevents them from being killed very quickly by the antibiotic? Explain how your response relates to this
activity on hand sanitizers.
Homework:
Hand sanitizers are used everywhere: homes, libraries, schools, grocery stores, hospitals, etc. Washing with regular soap
and water also helps get rid of bacteria as shown in the picture below.
There are advantages and disadvantages to using hand sanitizers over washing with soap and water. Describe two
reasons why hand sanitizers should be used instead of soap and water. Describe two reasons why hand sanitizers should
not be used. Use evidence from this activity to support your reasons.
Modeling natural selection
Materials (per group of three students)
1 paper cup containing 20 white Styrofoam packing peanuts
1 paper cup containing 8 skittles of any color
1 toothpick
1 8” diameter paper plate
1 clock or stopwatch
(Note: Do not eat the candy.)
1. Put eight Styrofoam packing peanuts and two Skittles on the same plate in the middle of your group. The Styrofoam
packing peanuts and Skittles represent bacteria. Most of the bacteria (Styrofoam packing peanuts) are the same (clones)
due to asexual reproduction. They have a soft “shell,” are white, and are cylindrical in shape. Two of the bacteria (the
Skittles) are not like the other clones. They represent bacteria that had three mutations resulting in three different traits
(colorful, hard shell, and disc shaped).
2. Look at how the beginning row in Table 1 is filled out.
3. The toothpick represents antibiotics. When you use the toothpick to catch the bacteria (packing peanuts and Skittles), it
is like bacteria being destroyed by the antibiotics
4.First dose of antibiotics (D1): One student in the group uses the toothpick to pick up—one at a time—and set aside as
many of the bacteria as possible in seven seconds.
5. The student then counts how many bacteria of each kind are left on the plate, and all students in the group fill in the
second row on the table (D1).
6.The first student then lets the remaining bacteria “reproduce by fission” by making copies of them. In other words, the
student doubles the number of any of the remaining bacteria. For example, if there are three packing peanuts left, make
six by adding three more packing peanuts to the plate. If there are two Skittles left, make four by adding two more Skittles
to the plate. This represents reproduction 1 (R1).
7. The first student then counts the number of each kind of bacteria on the plate after reproducing and all students in the
group fill in the third row on the table (R1).
8. The second student in the group repeats steps 4 and 5 for the second dose of antibiotics (D2), catching bacteria—one
a time—with the toothpick in seven seconds and counting the remaining bacteria. All group members fill in the fourth row
of the data table (D2).
9. The second student repeats steps 6 and 7 for the second reproduction (R2).
10. The third student in the group repeats steps 4 and 5 for the third dose of antibiotics (D3) and steps 6 and 7 for the
third reproduction (R3); all students record the data.